1. Field of the invention
The present invention relates to copolymers suitable for photoresists. More particularly, the present invention relates to a vinyl 4-tetrahydropyranyloxybenzal-vinyl 4-hydroxybenzal-vinyl tetrahydropyranyl ether-vinyl acetate copolymer and a vinyl 4-tetrahydropyranyloxybenzal-vinyl tetrahydropyranylether-vinyl acetate copolymer. Also, the present invention is concerned with methods for preparing the copolymers.
2. Description of the Prior Art
The demand for highly integrated semiconductor devices has made a significant advance for microlithography, one of the most essential techniques to achieve the high integration of a microelectronic devices. As a result, a variety of photoresists have now been developed. Among these photoresists, polyvinylphenol resins protected by t-butoxycarbonyl group, that is, poly(4-t-butoxycarbonyloxystyrene) resins, which are disclosed in U.S. Pat. Nos. 4,491,628, 4,405,708 and 4,670,507, are expected to be the most useful and are being used in practice.
In spite of significant advantages, these polyvinylphenol resins, however, are known to show high optical absorbance at around 250 nm due to the benzene rings contained therein. Because deep uv, a light source for the microlithography in current use, has a frequency of around 250 nm, the polyvinylphenol resins have low transparency at the frequency.
Poly(4-t-butoxycarbonyloxystyrene) resins lose much weight upon post-exposure baking in addition to being poor in adhesion. Like polyvinylphenol resins, they show low transparency because of high optical absorbance at around 250 nm. For example, a resin obtained by using 4-t-butoxycarbonyloxy styrene with an average molecular weight of 10,000 as a monomer and 2,2'-azobisisobutyronitrile (AIBN) as an initiator, shows a uv optical absorbance of 0.170 at 250 nm when it is 1 .mu.m thick. For a resin polymerized from the monomer in the presence of benzoylperoxide, its uv optical absorbance is 0.140 .mu.m.sup.-1. As exemplified above, the resins are both opaque at around 250 nm because of their high optical absorbance.
In order to solve the above problems, novel copolymers, vinyl 4-t-butoxycarbonyloxybenzal-vinyl alcohol-vinyl acetate copolymer and vinyl 4-t-butoxycarbonyloxybenzal-vinyl 4-hydroxybenzal-vinyl alcohol-vinyl acetate copolymer were suggested by the present inventors in Korean Pat. Appl'n No. 9-44162 the content of which is incorporated by reference. Though these novel materials are advantageous in many aspects, they may not be used for high temperature processing due to their relatively low glass transition temperatures.